libjavassist-java binary package in Ubuntu Focal amd64

 This library makes Java bytecode manipulation simple. It enables Java
 programs to define a new class at runtime and to modify a class file
 when the JVM loads it.
 .
 Unlike other bytecode editors, Javassist provides two levels of API:
 source level and bytecode level. With the source-level API, users can
 edit a class file without knowledge of the specifications of the Java
 bytecode. You can even specify inserted bytecode in the form of source
 text; Javassist compiles it on the fly. On the other hand, the
 bytecode-level API allows the users to directly edit a class file as
 other editors.

Source package